1. Field of the Invention
This invention relates to an image processing art, specifically to the control for compressing the current frame image and storing the compressed data in a shared memory and for reading and decompressing the compressed data of the adjacent frame image stored in the shared memory.
2. Description of Related Art
A variety of methods are proposed to decrease the use of memory and to prevent the posterior processing unit from overflowing at the time of compressing and decompressing image data.
For example, Patent Document 1 discloses the art of compressing image data according to the compression conversion rule corresponding to image data luminance information as a technology of reducing the use of memory and storing the compressed data and the compression conversion rule in the memory. At the time of decompression, the compressed data and the compression conversion rule are read from the memory, and the compressed data are decompressed according to the compression conversion rule. Patent Document 2 discloses, at the time of codifying image data and outputting the same to the processing unit of the subsequent stage (referred to as “VBV buffer” in Patent Document 2) through the transmission buffer memory, the art of monitoring the bit rate outputted from the transmission buffer memory, and controlling the data outputted by the transmission buffer memory when this bit rate has exceeded a predetermined value to reduce the output bit rate for preventing the overflow of the VBV buffer.
In an apparatus for handling the compression and decompression of image data, while the frame images successively inputted (referred hereinafter simply as “frame”) are compressed, the compressed data are stored in the shared memory, and the compressed data of the adjacent frame stored in the shared memory are read out and are decompressed constituting a frame transfer processing operation.
For example, in the liquid crystal display, an overdrive processing operation applying an overvoltage rather than a voltage that reaches the target voltage when a response has been completed due to the slow response of liquid crystal is known, and for this overdrive processing operation, the frame transfer processing operation described above is used (see Patent Document 3).
The overdrive processing operation is described below with reference to FIG. 7. FIG. 7 shows an image display apparatus 1. The image display apparatus 1 includes an image display unit 10 for performing an overdrive processing operation on frames successively inputted and a liquid crystal display unit 20 such as a liquid crystal display panel for displaying images corresponding to the frames processed by the image processing unit 10. The image processing unit 10 includes a compression unit 11, a shared memory 12, a decompression unit 13, and a compensation unit 14. The compression unit 11 compresses the current frames and store them in the shared memory 12, and the decompression unit 13 reads the adjacent compressed frame from the shared memory 12 and decompress the same to obtain the adjacent decompressed frame. The compensation unit 14 generates an amount of overdrive in the direction of changing from the data of the adjacent decompressed frame to the current frame data, and compensates the current frame data by this amount of overdrive and output the same to the liquid crystal display unit 20.
Such an image processing apparatus includes normally not only a processing unit charged with compression processing and decompression processing but also another processing unit for accessing the shared memory. In a SoC (System-on-a-Chip) in which a plurality of processing units use the shared memory and the like, it is important to manage the bandwidth of the memory bus for accessing the shared memory. This is because, if a processing unit uses excessively the memory bus bandwidth, there develops a problem of memory bus breakdown in that other units will be unable to process.
Patent Document 4 discloses the art of restricting the capacity of the shared memory and preventing the deterioration of image quality in an image display apparatus having an overdrive function. This art divides the object frame in a plurality of blocks at the time of compression, and compresses each block by adjusting the compression ratio so that the data amount will be such that corresponds to the amount of memory used that has been predetermined for this block, and the compressed data of each block and the accuracy information showing the compression ratio for the block (referred to as “OD accuracy information” in patent documents) are stored in the shared memory. At the time of decompression, the compressed data and the OD accuracy information of each block of the object frame are read from the shared memory, and based on the OD accuracy information read out, the compressed data of each block of the object frame are decompressed to obtain the decompressed frame. At the time of generating the overdrive quantity for the current frame, the overdrive quantity for pixels in the corresponding block is generated according to the OD accuracy information of each block of the decompressed frame of the adjacent frame.
Patent Document 5 calculates the transfer quantity data of transferring common bus from the operational bus information in the processing mode for each processing in the system in which a plurality of modules of the image coder/decoder unit transfer data by using the shared bus, and changes the coding parameters of the image coder/decoder unit according to the quantity of transferred data.    [Patent Document 1] Japanese Patent Laid Open Application No. 2005-311743    [Patent Document 2] Japanese Patent Laid Open Application No. 2006-180036    [Patent Document 3] Japanese Patent Laid Open Application No. 2001-265298    [Patent Document 4] Japanese Patent Laid Open Application No. 2006-267172    [Patent Document 5] Japanese Patent Laid Open Application No. Hei 10(1998)-224532
The art disclosed in Patent Document 4 has a problem in that it can reduce the memory used by the compression processing unit and consequently the memory bus bandwidth by raising the compression ratio but that the image quality deteriorates when the compression ratio is reduced. There is a compression algorithm with variable amount of compressed data even with the same compression ratio, for example reversible compression, and in the case of compression with such a compression algorithm, it is necessary to allocate the amount of memory used to each block by assuming in advance the maximum amount of compressed data, and therefore waste can result. If an overdrive quantity is generated according to the OD accuracy information showing the compression ratio of each block, then only variation in the image quality caused by the compression of a plurality of blocks within the same frame with different compression ratios can be dissolved, and the deterioration of image quality caused by a high compression ratio cannot be dissolved.
The art of Patent Document 5 which consists in changing coding parameters of image data in order to prevent the breakup of the shared bus is like Patent Document 4 plagued by the problem of deterioration of image quality caused by changes in coding parameters. This art essentially calculates the bus bandwidth used by various processing units depending on the processing mode, and based on the result of that calculation allocates a predetermined bus bandwidth to the image coder/decoder unit. For example, even when other processing units use less bus bandwidth than calculated, the bus bandwidth opened by the image coder/decoder unit cannot be used, and as a result efficiency is low.